>> Can someone confirm or deny that the DACs in Atari color games, let's
>> say Star Wars at least, are setup to produce a change in "current"
>> rather than a change in "voltage" at their output?
>
>Yes. I recall seeing that in 1 or 2 schematics. The DAC outputs a current
>which is then fed to an OP-Amp to get a voltage. It's important to
>have a voltage, as that's what matters to the analog switch...
Paul's right. Atari uses current output DACs.
>As I recall, the output of the DAC is not the beam position, but rather
>the velocity. To get beam position, there is another op-amp configured
>as an integrator - apply the "velocity" voltage and the position responds
[...]
>in signals from somewhere else - like a cinematronics game - if you feed
>them in at that point, you get to use the Atari pincousion stuff. Just a
>thought...
>
>This is all from memory, so there may be something in error. Can anyone
>think of a correction?
Right, Atari used the LF13201 analog switches to short the integrator caps
resulting in a "return to center of screen" action for the AVG based
systems (like Star Wars). Like Paul said, the analog switches were also
used to route the voltage output through another inverting amplifier to
produce a screen "flip" for the cocktail games.
On older DVG based games (like Asteroids), the analog switches were not
needed for integrator control (the vectors were directly generated by the
DVG), but were instead used for "cocktail flip" as described above.
Kind-of interesting to note that Atari would leave a lot of things
"unpopulated" on some boards (presumably to cut costs), but never bothered
to customize loading options for the PCB's for non-cocktail versions of the
vector games... (I'd have to guess that they mostly shipped uprights, so
it seems odd to burden the volume shipper with extra hardware costs...)
-Clay
Clayton N. Cowgill Engineering Manager
_______________________________________________________________________
/\ Diamond Multimedia Systems, Inc. clay@supra.com
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Received on Mon Jan 19 16:14:26 1998
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